Artificial Intelligence in Game Design Goal-Oriented Behavior.

Artificial Intelligence in Game Design

Goal-Oriented Behavior

Goals

• Complex characters have goals– Add to realism of character

• Actions appear motivated instead or reflexive

• Player must be aware of character motivations

– Example: Napoleon “boss” NPC• Capture other cities

• Protect own cities

• Increase gold level

• Issue:– What if goals conflict?

• Get gold

• Avoid player???

Goals and the Sims

• Overall goals (player meets)– Positive and negative goals

• Goals based on physical needs– Hunger– Energy– Fun – Etc.

Characters automatically choose actions to meet needs

Goals and Actions

• Goals determine actions than fulfill need – Game keeps list of actions for each need

– Which actions should NPC take to fulfill goals?

Hunger Eat snackCook dinner

Energy Nap in chairSleep in bed

Fun Go to concertPaintRead

Insistence of Needs

• Needs may have “degree of urgency” (insistence)– How critical is need?– How important is this need relative to others?– How much of an action is needed to fulfill need?

• Represented as number– Higher = greater need– Zero = no need

Need Insistence

Hunger 5

Energy 0

Fun 3

Actions and Goals

• Actions can permanently fulfill goals– Replaced with another related goal (Sims)

– Ends game (Napoleon captures all cities in world)

• Actions can temporarily lessen needs– Action: Need – n

lessens that need by n

Hunger Eat snack: Hunger – 1 Cook dinner: Hunger – 5

Energy Nap in chair: Energy – 1 Sleep in bed: Energy – 5

Fun Go to concert: Fun – 4 Paint: Fun – 2 Read: Fun – 1

Choosing Actions

• Simple approach:– Choose goal with highest insistence– Choose action that gives largest decrease

Need Insistence

Hunger 6

Energy 0

Fun 3

Eat Eat snack: Hunger – 1 Cook dinner: Hunger – 5

Energy Nap in chair: Energy – 1 Sleep in bed: Energy – 5

Fun Go to concert: Fun – 4 Paint: Fun – 2

Greatest need

Best action

Side Effects

• Actions can effect multiple goals

Drink Coffee Hunger – 1 Energy – 1

Eat at Restaurant

Hunger – 4 Fun – 3

Gook Gourmet Meal

Hunger – 5 Fun – 1Energy + 1

Side effect

Fulfilling one goal increases other needs

Overall Utility

• Utility-based decision making– Determine effect of each action on total needs

– Choose action that has best overall effect

– One criteria: sum of all needs

Need Current level

After Cook Gourmet Meal

After Drink Coffee

After Eat at Restaurant

After Sleep in Bed

Hunger 3 0 2 0 3

Energy 2 3 1 2 0

Fun 4 3 4 1 4

Total effect 6 7 3 7

Best action

Overall Utility

• Problem: This can ignore very severe needs– Behavior does not appear rational!

Need Current level

After Eat at Restaurant

After Sleep in Bed

Hunger 3 0 3

Energy 10 10 5

Fun 3 0 3

Total effect 10 11

Will be chosen, even though Sim is very tired!

Overall Squared Utility

• Common solution:Total discontentment = sum of square of needs

Need Current level

After Eat at Restaurant

After Sleep in Bed

Hunger 3 02 32

Energy 10 102 52

Fun 3 02 32

Total effect 100 43

Planning and Preconditions

• Actions may have preconditions before they can be taken

• Multiple actions required to meet needs

ActionPreconditions Postconditions

ActionPreconditions which character meets

Postconditions

ActionPreconditions Postconditionsmeet need

meets

Planning and Preconditions

• Example: Microwaving food

Get foodUncooked food in fridge

Have uncooked food

Cook food in microwave

Have uncooked foodHave microwave

Have cooked food

meets

Eat foodHave cooked food

Hunger - 5

meets

Planning Methods

• Scripted action sequences– Actually single action with what appears to be multiple steps

– Long animated sequence

• Very simple to implement (no planning needed!)• Weaknesses:

– Cannot interrupt sequence without starting over entirely• Phone rings while cooking

– Cannot start plan until all preconditions met

Get foodCook food in microwave

Eat food

Have uncooked foodHave microwave

Hungry - 5